For x-ray imaging applications, x-rays are often generated by a transmission-target x-ray generator having a schematic configuration as shown in FIG. 1.
The x-ray generator 100 shown in FIG. 1 includes an electron-beam generator 110 which generates an electron beam travelling in the direction indicated by arrow Be. The electron beam strikes plate-like target 120 made of a high-Z (high atomic number) material such as tungsten, such that x-rays are emitted from the material. The principal intended direction of emission of the x-rays is shown by schematic arrow Bx in FIG. 1, although this arrow in reality only indicates an axis of symmetry for the x-ray generation since the x-rays are emitted in a relatively large range of angles to the incident electron beam direction Be, although emission in the sideways and reverse directions is supressed to some extent by absorption of the x-rays in the target 120. The x-ray beam has a characteristic energy spectrum which depends on both the material from which target 120 is made and the energy distribution of electrons in the incident electron beam.
The configuration of the x-ray generator shown in FIG. 1, being a transmission-target configuration, is thus distinct from a reflection-target configuration, which uses a relatively thicker target and in which the intended direction of emission of the x-rays is at an angle greater than 90 degrees to the incident electron beam direction Be to the surface of the target.
Both the electron beam generator 110 and target 120 are enclosed in vacuum enclosure 140, since the presence of matter inhibits the transmission of the electron beam. Vacuum enclosure 140 is generally not transparent to x-rays, so is provided with an x-ray emission window 130 positioned downstream of the target 120, i.e. on the opposite side to the electron beam generator 110, in the intended direction of emission of the x-rays Bx. The window 130 is made of a material which is relatively transparent to x-rays, i.e. having a low radiodensity and being relatively thin. Therefore, x-rays generated in target 120 which impinge upon window 130 are able to pass through window 130 and exit the apparatus. X-rays generally easily pass through air and other gases, so the x-ray beam is not significantly attenuated after passing through window 130. Window 130 is commonly made of beryllium, which has a very low radiodensity relative to other materials.
Since x-rays are generated in target 120 at a range of angles to the electron beam direction Be, it is necessary to reduce the angular spread of the beam sufficient to avoid unintended irradiation of objects near to the beam path. Typically, this is achieved by means of a collimator 150, which provides a layer of x-ray absorbing, i.e. radiodense, material positioned in the x-ray beam having emerged from window 130, the layer having a central aperture through which the x-rays can pass. X-rays which do not pass through the aperture are absorbed in the radiodense material, the eventual angular spread of the resultant beam being determined by the diameter of the aperture and the distance of the collimator 150 from the target 120.
Herein, reference has been made to radiodensity as a property of materials determining their ability to transmit x-rays. Radiodensity may be measured, for example, by the Hounsfield scale, in which distilled water has a value of zero Hounsfield units (Hu) while air has a value of minus 1000 Hounsfield units (Hu). Relative radiodensity does not significantly very with x-ray energy, but may, for example, be measured or calculated with a characteristic x-ray beam energy of 200 keV.
In arrangements such as shown in FIG. 1, it has been noticed that ozone is sometimes generated by such an x-ray source. The presence of ozone is of concern to both manufacturers and users. Therefore, there is a need to suppress the production of ozone in such x-ray apparatus.